Theories in science are "proven" only to the extent that the results of the studies used to test theories can reproduce the same results. Scientists construct a hypothesis and then test that hypothesis with experiments. If the results of the experiments disprove the theory, that gives the scientist new information. The scientist then uses that information to revise his hypothesis and test his or her revised theory.
hope this helps some

see... scientists usually perform an experiment, with what they observe in nature...and to prove them they use experiments...and if the result of their experiment is concurrant to that of the observed phenomena, that means their assumption was correct, whereas even if they are wrong, they are able to uncover some ambiguous, hidden fact which can change our perspective towards that phenomena :P...sheesh hope this is understandable :P

For the same reason that your theory of how to study, or why your bank balance is strangely dropping, or whether or not your best friend is truly a friend, et cetera, can be modified if you acquire new evidence supporting or discrediting them.